Design of an imaging spectrometer for earth observation using freeform mirrors

Design of an imaging spectrometer for earth observation using freeform mirrors Thomas Peschel1, Christoph Damm1, Matthias Beier1, Andreas Gebhard1, Stefan Risse1, Ingo Walter2, Ilse Sebastian2, David Krutz2 1 Fraunhofer Institut fur Angewandte Optik und Feinwerktechnik, Jena 2 DLR, Institut fur Optische Sensorsysteme, Berlin In 2017 the new hyperspectral DLR Earth Sensing Imaging Spectrometer (DESIS) will be integrated in the Multi-User-System for Earth Sensing (MUSES) platform /1/ installed on the International Space Station (ISS). The DESIS instrument is developed under the responsibility of the DLR. It will deliver images of the earth with a spatial resolution of 30 m on ground in 235 spectral channels in the wavelength range from 400 nm to 1 µm. As partner of the development team Fraunhofer IOF is responsible for the optical system of the imaging spectrometer.The optical system is made of two primary components: A compact Three-Mirror-Anastigmat (TMA) telescope images the ground strip under observation onto a slit. The following spectrometer reimages the slit onto the detector and performs the spectral separation using a reflective grating. The whole optical system is realized using metal-based mirrors the surfaces of which are made by Single-Point-Diamond Turning (SPDT). Since the spectral range is in the visible, a post-processing of the surfaces by Nickel plating is necessary. The final surface shape and roughness are realized by a second SPDT step and subsequent Magneto-Rheological Finishing. The TMA provides a focal length of 320 mm and an aperture of F/2.8. Its mechanical design relies on the Duolith-technology of IOF as well as optical and mechanical reference structures on the mirrors /2/ manufactured in the same SPDT run. This strategy allows for a significantly simplified adjustment of the optical system /3/. The spectrometer was designed on the basis of the so-called Offner scheme. Because of the high aperture of the system a freeform mirror had to be introduced in order to provide a good imaging quality over the whole spectral range. The above optical design requires a grating on a curved surface. Technologies are developed in order to fabricate the grating either by SPDT or, alternatively, by laser lithography. The mechanical design uses light-weight housing elements which wrap the optical path to suppress stray light. An athermal design is provided by using the same metal for mirrors and housing. To provide high adjustment precision, the housing elements carry reference and mounting features made by SPDT as well. This approach allows for a stiff mechanical set-up of the system, which is compatible with the harsh requirements of a space flight. References: 1 N. Humphrey, “A View From Above: Imaging from the ISS”, Teledyne DALSA 2014, http://possibility.teledynedalsa.com/a-view-from-above/ 2 S. Scheiding, e.a., “Ultra-precisely manufactured mirror assemblies with well-defined reference structures“, Proc. SPIE 7739, 2010. 3 T. Peschel, e.a., “Anamorphotic telescope for earth observation in the mid-infrared range”, ICSO 2014